Layered Architecture
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Most commonly used of all architectures
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Forms the basis of the modern paradigm of computing
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Modules use services of the module(s) directly below it
and provide services to the module(s) above it
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Promotes loose coupling of cohesive modules
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Often used as a "fallback" architecture when an obvious
solution is not present
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Other architectures can use this style, in a hybrid
fashion
Layered Architecture
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Aqua Lush Example
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LCD TV Example (Bai and Chen)
High-Level Diagrams
Wedding Cake Diagram
Layer 5
Layer 4
Layer 3
Layer 2
Layer 1
Onion Diagram
Layer 5
Layer 4
Layer 3
Layer 2
Layer 1
Wedding Cake Design is typically used to show connection between
layers in communication protocols and user interfaces.
Onion Diagram typically illustrates operating system layers.
Communication
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Static Structure:
The software is partitioned into layers that provide a cohesive set of
services with a well-defined interface.
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Dynamic Structure:
Each layer is allowed to only use
 the layer directly below it (Strict Layered style)
 all the layers below it (Relaxed Layered style)
(Wedding Cake) Layered Architecture for
Office Delivery Robots
This example uses Strict Style.
Four Main Layers
 Task Scheduling, Path Planning, Navigation and Obstacle Avoidance
 Task Scheduling determines how long all the routes will take
(determine by asking Path Planner the length of each path).
 Path Planner creates paths the robot can take (Factoring in what
navigation has said is possible).
 Navigation decides what paths can be taken to reach destination
(Using data from Obstacle Avoidance).
 Obstacle Avoidance decides how to handle each obstacle the robot
may run into.
Advantages for the Layered Style
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The layers can make great modules
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Helps simplify the program by sub-dividing complex
portions
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Layers make use of the services provided by the layer
below.
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Layers provide services to the layer above
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Satisfies the Principle of Cohesion
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Layers are loosely coupled
Advantages for the Layered Style
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Supports information hiding
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Layers help increase changeability, reusability,
reliability, and maintainability
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Changes can be made to certain layers and not the core
functionality of the program
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Systems can be easily ported to new platforms
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Layers provide well-defined interfaces that can have
reusable components
The Problem with Overhead
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Allocating information on a system with multiple sub-levels can become a
problem in Layered Architecture design
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Data that is passed through multiple layers can sometimes be changed due
to the encryption and decryption of certain levels when either reading or
writing information
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Performance becomes an issue when traveling through multiple layers
instead of accessing a component directly
Restrictions to Layered Independence
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Shared processes that are modified or added within higher layers
must also be modified in the lower levels associated with the higher
level
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Modifying lower-level interfaces sometimes percolates to higher
levels, commonly seen in a relaxed layered approach
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Applications that require interface components can have longer
standby times if the layers prevent them from accessing a database
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Large applications can be controlled and encapsulated more
effectively but this adds complexity to smaller systems
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Satisfying an n amount of layers